Much of modern electronic assembly is automatic. A robot with specially fabricated grasping fingers grasps a selected electronic component from a dispensing station. This component may be a flat pack with a plurality of flat leads extending from one or more sides thereof or may be a cylindrical component such as a resistor having round wire leads thereon. The leads of the components are shaped in such a manner that they lie at or below the bottom surface of the component for surface mounting on the printed wiring board. After a component is grasped, the component may be optically inspected so that it is the correct component for this particular assembly step. Next, the component may have its leads fluxed, and thereafter the component is put in place on the printed wiring board. When it is put in place, its leads are located on solder pads which have sufficient solder thereon for reflow soldering to the leads on the component. When placed, heater bars are brought over the component leads and are brought down on the leads to hold them in place. While held in place, the heater bars heat the leads and the adjacent solder to reflow solder the leads in place. The assembly is complete, except for the inspection of the adequacy of the solder joint. Present inspection is visual. The inspection is by employment of microscopes having magnification of under four times to microscopes having magnification of seven times. Prior art lighting is usual microscope illumination, about 45 degrees to the perpendicular to the surface of the printed wiring board. This is the best quality control inspection presently available, but its disadvantages include the subjectivity of the inspection and the personal judgment required. This is compounded by limited visual acuity and limited attention span. The article must be manually moved with respect to the microscope which can cause hand-eye coordination errors. Each of the errors increases with fatigue as the work day goes on. As a result of this, an accept-reject decision cannot be repeated with 100 percent repeatability. In addition, such inspection methods are labor intensive and require expensive training together with periodic certification of the trained inspectors.
There have been attempts to automatically inspect by means of laser illumination together with infra-red signature sensing and analysis technique. The problem with the infra-red inspection technique is that it cannot detect all visual defects. It is not as sensitive to problems as a good visual inspection. It cannot detect extraneous solder, away from the solder pads. It cannot detect subtle visual solder defects such as insufficient solder or dewet conditions, where the solder does not wet both surfaces of the intended solder joint. Further, it cannot detect misalignment between the leads and the pads. Other attempts at automatic inspection include the use of two colors of light at different illumination angles, but such have not been reliable to date. Thus, present-day automatic inspection has not been satisfactory.